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    SubjectRe: [PATCH v5 00/45] CPU hotplug: stop_machine()-free CPU hotplug
    On 02/08/2013 10:14 PM, Srivatsa S. Bhat wrote:
    > On 02/08/2013 09:11 PM, Russell King - ARM Linux wrote:
    >> On Thu, Feb 07, 2013 at 11:41:34AM +0530, Srivatsa S. Bhat wrote:
    >>> On 02/07/2013 09:44 AM, Rusty Russell wrote:
    >>>> "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com> writes:
    >>>>> On 01/22/2013 01:03 PM, Srivatsa S. Bhat wrote:
    >>>>> Avg. latency of 1 CPU offline (ms) [stop-cpu/stop-m/c latency]
    >>>>>
    >>>>> # online CPUs Mainline (with stop-m/c) This patchset (no stop-m/c)
    >>>>>
    >>>>> 8 17.04 7.73
    >>>>>
    >>>>> 16 18.05 6.44
    >>>>>
    >>>>> 32 17.31 7.39
    >>>>>
    >>>>> 64 32.40 9.28
    >>>>>
    >>>>> 128 98.23 7.35
    >>>>
    >>>> Nice!
    >>>
    >>> Thank you :-)
    >>>
    >>>> I wonder how the ARM guys feel with their quad-cpu systems...
    >>>>
    >>>
    >>> That would be definitely interesting to know :-)
    >>
    >> That depends what exactly you'd like tested (and how) and whether you'd
    >> like it to be a test-chip based quad core, or an OMAP dual-core SoC.
    >>
    >
    > The effect of stop_machine() doesn't really depend on the CPU architecture
    > used underneath or the platform. It depends only on the _number_ of
    > _logical_ CPUs used.
    >
    > And stop_machine() has 2 noticeable drawbacks:
    > 1. It makes the hotplug operation itself slow
    > 2. and it causes disruptions to the workloads running on the other
    > CPUs by hijacking the entire machine for significant amounts of time.
    >
    > In my experiments (mentioned above), I tried to measure how my patchset
    > improves (reduces) the duration of hotplug (CPU offline) itself. Which is
    > also slightly indicative of the impact it has on the rest of the system.
    >
    > But what would be nice to test, is a setup where the workloads running on
    > the rest of the system are latency-sensitive, and measure the impact of
    > CPU offline on them, with this patchset applied. That would tell us how
    > far is this useful in making CPU hotplug less disruptive on the system.
    >
    > Of course, it would be nice to also see whether we observe any reduction
    > in hotplug duration itself (point 1 above) on ARM platforms with lot
    > of CPUs. [This could potentially speed up suspend/resume, which is used
    > rather heavily on ARM platforms].
    >
    > The benefits from this patchset over mainline (both in terms of points
    > 1 and 2 above) is expected to increase, with increasing number of CPUs in
    > the system.
    >

    Adding Vincent to CC, who had previously evaluated the performance and
    latency implications of CPU hotplug on ARM platforms, IIRC.

    Regards,
    Srivatsa S. Bhat



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